High Water Mask（最高水位線，呵呵）以下簡稱HWM
delete命令不影響表自身的HWM，即表中的區塊會隨着數據量的增大而增大，但刪除數據時，
區塊並不會減少，當查詢時，會按照HWM的標準去掃描表，即使使用delete將表數據清空，查詢時，
依舊會掃描HWM的區塊，查詢速度也就和沒使用delete命令之前是一樣的，而truncate可以將表的
HWM重置。
999999條數據時，統計個數的執行計劃
SQL>
1* select count(1) from td_count
Execution Plan
---------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'TD_COUNT'
Statistics
---------------------------------------------------------
0 recursive calls
0 db block gets
1004140 consistent gets
10008 physical reads
107380 redo size
380 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
/************************************************************/
執行delete刪除所有數據後的執行計劃
SQL>
1* select count(1) from td_count
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'TD_COUNT'
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
2004139 consistent gets
23703 physical reads
107380 redo size
380 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
/************************************************************/
執行truncate後的執行計劃
SQL> select count(1) from td_count;
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'TD_COUNT'
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
380 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed